1. Field of the Invention
The present invention relates to a data conversion apparatus, a signal, a data conversion method, a DCE (Data Circuit-terminating Equipment), a gateway and a communication apparatus. More specifically, the present invention relates to a data conversion apparatus etc. for inhibiting octet/bit insertion in a section which does not require octet insertion or bit insertion (hereinafter referred to “octet/bit insertion”) during communication on the basis of PPP (Point-to-Point Protocol). The present invention further relates to a communication apparatus which reduces control packet on the communication line.
2. Description of the Related Art
PPP is present as a data link layer protocol of OSI reference model. PPP is a bit/byte synchronous and an asynchronous link control protocol by a serial circuit. PPP is specified in RFC (Request For Comments) 1661 and RFC 1662 of IETF (Internet Engineering Task Force).
FIG. 1 is a diagram showing a PPP frame configuration (frame format). The PPP frame has a flag field, address field, control field, protocol field, information field and FCS field. The number of bytes of respective fields are 1 byte for the flag field, 1 byte for the address field, 1 byte for the control field, 1 or 2 bytes for the protocol field, and 2 or 4 bytes for the FCS field. The address field and the control field may sometimes be compressed by negotiation of LCP (Link Control Protocol) ACFC (Address and Control Field Compression). Further, the protocol field may sometimes be compressed by LCPPFC (Protocol Field Compression) negotiation. Still further, the FCS (Frame Check Sequence) field may sometimes be compressed by LCPFCS negotiation.
FIG. 2 is a diagram showing an example of communication between DTE (Data Terminal Equipment) through a communication network and PSTN (Public Switched Telephone Network). In FIG. 2, an exchange 8 and a gateway 10 forms a communication network 5. DTE 2 and DTE 14 perform communication through DCE 4, exchange 8, gateway 10 and PSTN12. The communication network 5 can be, for example, a mobile communication network, and DCE 4 be, for example, a mobile station.
In this case, it is considered that data communication between DTE 2 and DTE 14 is performed on the basis of PPP. Heretofore, when a data signal is transmitted from DTE 2 to DTE 14, the data signal has been transmitted after making octet/bit insertion in DTE 2. Further, in DTE 14, octet deletion or bit deletion (hereinafter referred to “octet/bit deletion”) has been performed to the received data signal. On the other hand, when a data signal is transmitted from DTE 14 to DTE 2, octet/bit insertion has been performed in DTE 14, and octet/bit deletion in DTE 2.
Further, it is also considered that only data communication between DTE 2 and a given point (point for terminating PPP) in the network is performed on the basis of PPP. As the given point in the network, for example, the exchange 8, the gateway 10 or the like is considered. Here, description is made for a case where the gateway 10 is the terminating point. In the past, when a data signal is transmitted from DTE 2 to DTE 14, octet/bit insertion has been made in DTE 2, octet/bit deletion made on the received data signal in gateway 10, and then the data signal has been transmitted to DTE 14. On the other hand, when transmitting a data signal from DTE 14 to DTE 2, octet/bit insertion has been made in gateway 10, and octet/bit deletion has been made in DTE 2.
However, in data communication based on the prior art PPP, even in a section not requiring octet/bit insertion, a data signal is transmitted and received in the octet/bit inserted state. For example, it is considered that octet/bit insertion is unnecessary in the section between DCE 4 and gateway 10 in FIG. 2, however, in the prior art, octet/bit insertion has been made even in this section. When octet/bit insertion is made, data transmission amount is increased, and throughput is deteriorated. Octet insertion means, for example, a specific 1-byte data is escape processed with a 1-byte escape character (1 byte) to form a 2-byte data (will be described later in detail with reference to FIG. 9). Further, bit insertion is described using an example, when a flag (“01111110” in binary notation) is to be distinguished from other data portion, bit insertion is made, to data other than the flag, when five “1s” continue, “0” is inserted after the data.
Incidentally, as a DTE data link layer protocol used for the purpose of connecting to internet/intranet through public communication network or private communication network, PPP (Point-to-Point Protocol) is frequently used.
PPP enables transfer of various network protocols such as IP (Internet Protocol), Appletalk, and the like. The PPP specifications are specified as RFC (Request For Comment) of IETF (Internet Engineering Task Force).
FIG. 17 is a diagram showing a communication example. In the example of FIG. 17, a DTE (Data Terminal Equipment) 52 and a DTE 60 perform communication through a DCE (Data Circuit-terminating Equipment) 54, a network 56 and a DCE 58. Here, the DCE 54 and the DCE 58 can be, for example, mobile stations (portable telephones).
FIG. 18 shows a PPP frame format. Flag represents start or end of PPP, and is used for identifying a frame partition. Address field is information representing address of this PPP frame, for which a fixed value is generally used. Control field is information used for identifying frame type, for which a fixed value is generally used. Protocol field is used for identifying the protocol of packet included in the information field, and protocol packet shown in the protocol field is included in the information field. FCS (Frame Check Sequence) is used for making error detection from the address field to the information field.
FIG. 19 shows a PPP procedure. PPP transfers to a link establish phase when a physical layer starts in a link stop phase. In the link establish phase, link setting procedure of LCP (Link Control Protocol) is carried out, after LCP link establishment, transfers to a verification phase, where a verification procedure is carried out as necessary. When verification is successful, PPP transfers to a network layer protocol phase, where NCP link setting procedure corresponding to each network protocol is performed. When each NCP link is established, packet of network protocol corresponding to each NCP becomes transferable. Further, PPP transfers to a link end phase by a communication end request or the like. LCP instructs to transmit an end request packet, when its identification packet is received, so that PPP link is closed and the physical layer is cut off. Further, close of the PPP link is notified to the network layer. When the physical layer is cut off, PPP returns to the link stop phase.
A LCP or NCP link establish sequence example is shown in FIG. 20. Here, link establish condition is that node A and node B transmit and receive respective setting identification packets. In respective packets, ID is used for correspondence of setting request packet with response packet (setting identification packet, setting negation packet, or setting rejection packet), and ID value received in the request packet is included in the ID value of the response packet. When transmitting a setting request packet, an option (Opt_A˜G) can be designated.
Reception side of the setting request packet, when all options included in the setting request packet can be recognized, and those values are all tolerable, accepts all options included in the setting request packet in the setting identification packet and performs a response.
The setting request packet reception side, when an unrecognizable option is present in options included in the setting request packet, in the setting rejection packet makes a response including the unrecognizable option.
The setting request packet reception side, when all options included in the setting request packet are recognizable, however, intolerable option values exist, in the setting negation packet includes only intolerable value option, however, changes these options to tolerable values and performs a response. Further, when options other than those included in the setting request packet are to be requested to the setting request packet transmission side, those options can also be added to the packet.
The setting request packet transmission side, when receiving a setting rejection packet, such rejected options are removed, and then transmits the setting request packet again.
The setting request packet transmission side, when receiving a setting negation packet, a setting request packet with option values changed to those included in the setting negation packet. However, when a plurality of values exist in the options of the setting negation packet, one of them is selected.
A negotiation procedure example up to LCP or NCP link establishment will be described with reference to FIG. 20.
(a) Setting request (configure request) packet was transmitted from node A to node B, however, lost halfway in the transmission.
(b) Node A, because a response packet to setting request packet of (a) is not received for a certain period of time, transmitted the setting request packet again. At this moment, only ID value was set to a value different from the setting request packet of (a).
(c) Setting request packet was transmitted from node B to node A, however, lost halfway in the transmission.
(d) Node B, because options Opt_C, Opt_D, and Opt_E in the setting request packet of (b) cannot be recognized, made a response including these options in the setting rejection (configure reject) packet.
(e) Node A, removed options Opt_C, Opt_D, and Opt_E in the received setting rejection packet of (d), changed the ID value, and then transmitted setting request packet.
(f) Node B, because all options in the received setting request packet of (e) were recognizable and these values are all tolerable, in the setting identification (configure ack (acknowledge)) packet made a response including all options in the setting request packet.
(g) Node B, because a response packet to setting request packet of (c) is not received for a certain period of time, transmitted again the setting request packet of the same format as the setting request packet of (c).
(h) Node A, because option Opt_G in the setting request packet of (g) cannot be recognized, made a response including this option in the setting rejection packet.
(i) Node B, removed the option Opt_G in the received setting rejection packet of (h), changed the ID value and transmitted the setting request packet.
(j) Node A, because value w2 of the option Opt_A in the setting request packet of (i) is tolerable, however, value z1 of the option Opt_F is intolerable, and it is tolerable if the value is z2, in the setting negation (configure nak (negative acknowledge)) packet changed the value of option Opt_F to z2 and transmitted it.
(k) Node B, changed option Opt_F in the received setting negation packet of (j) and transmitted the setting request packet.
(l) Node A, because all options in the received setting request packet of (k) can be recognized and these values are all tolerable, in the setting identification packet made a response including all options in the setting request packet.
An example of LCP link cut sequence is shown in FIG. 21.
(a) Node A transmitted end request (terminate request) packet to require link release.
(b) On receiving the end request packet, node B transmitted the end identification (terminate ack) packet. Node A receiving the end identification packet became a link close state.
(c) Node B, after waiting for a certain time from the end identification packet transmission, transmitted an end request packet.
(d) Node A, on receiving the end request packet, transmitted the end identification packet, cut off the physical layer, and transferred to the link stop phase. The node B receiving the end identification packet became a link close state, cut off the physical layer, and transferred to the link stop phase.
FIG. 22 shows a keep alive sequence example using LCP echo request/response packet.
The LCP echo request/response packet is used during LCP link establishment and can be used for determining whether or not the link is held.
(a) Node A transmitted LCP echo request packet to confirm whether or not LCP link is maintained.
(b) Node B on receiving the LCP echo request packet, transmitted LCP echo response (echo reply) packet to respond that the link is maintained.
(c) Node B transmitted LCP echo request packet to confirm whether or not LCP link is maintained.
(d) Node A on receiving the LCP echo request packet transmitted LCP echo response packet to respond that the link is maintained.
As one of characteristics of packet communication, since the communication line is used only when data is generated, a communication node exists in which adoption of a communication fee system based on the data amount is possible. As a requirement in such a communication node, it is desirable that communication fee is not required when user data to be communicated does not exist, that is, actually the communication line is not used.
In the past, when PPP is used as DTE data link layer protocol in a communication node adopting a communication tariff system based on the data amount, by transferring PPP control packets at the communication start time and communication end time, LCP echo request/response packet for making continuity confirmation of PPP link periodically, and the like, there are problems that an extra communication tariff is required from the user point of view, and communication cost and communication traffic amount tend to increase from the communication enterprise point of view.